1. Field of the Invention
The present invention relates to a stencil printing machine of an inner press system in which a printing pressure is exerted from the inner circumference side of a printing drum, and also relates to a technique for preventing ink leakage from the sides of the printing drum (in the center axis direction of the printing drum).
2. Description of the Related Art
As shown in FIGS. 1 and 2, a printing section of a stencil printing machine is provided with a printing drum 100 and a pressure drum 101, and the printing drum 100 and the pressure drum 101 are respectively installed so as to freely rotate, with respective portions of their outer circumferential faces being closely located with each other. The printing drum 100 is provided with a pair of cylindrical flanges (not shown) aligned face to face with each other with a predetermined gap, and a stencil sheet clamp section 100a, which clamps the leading edge of a stencil sheet 104, is placed on a part of the outer circumferential face of each flange. A flexible screen 102 is stretched over the outer circumferential faces of the flanges of the printing drum 100 other than the stencil sheet clamp section 100a.
As shown in FIGS. 3 and 4, the screen 102, which forms a circumferential wall of the printing drum, consists of an area which is subjected to a pressing force applied by an inner pressing roll 106 and which is constituted by a rough mesh screen section 102a and a dense mesh screen section 102b that are overlapped with each other so as to form a pressing-time ink passage section 112 (indicated by a diagonal hatched portion in FIG. 3) through which ink 103 is only allowed to pass upon application of the pressing force, and an area which is not subjected to the pressing force and which is formed by the rough mesh screen section 102a to which a coating material is injected so as to provide an ink non-passage section 113 (indicated by a cross hatched portion in FIG. 3) through which no ink 103 is allowed to pass even upon application of the pressing force. All the circumferential portion of the dense mesh screen section 102b is affixed to the rough mesh screen section 102a through a bonding section 102c formed by using the coating material injected to the rough mesh screen section 102a as a bonding agent. On the inner circumferential face of the screen 102 as well as on the ink non-passage section 113 located on one of the outer circumferential sides of the pressing-time ink passage section 112, a raised portion 114a is formed, and on the other outer circumferential side that is rotation-delay side of the ink non-passage section 113 of the pressingtime ink passage section 12, a raised portion 114b is formed. In other words, on the Inner circumferential face of the screen 102, the raised portions 114a and 114b are formed in a U-letter shape.
Moreover, an inner press mechanism 105 is installed inside the screen 102 forming the circumferential wall of the printing drum 100. The inner press mechanism 105 is provided with an inner pressing roll 106, and this inner pressing roll 106 is formed on a roll support member 107 so as to freely rotate thereon. This roll support member 107 is supported so as to freely pivot centered on a support shaft 108 so that the inner pressing roll 106 is allowed to shift between a pressing position at which the inner pressing roll 106 is allowed to press the inner circumferential face of the screen 102 with the roll support member 107 being pressed in the direction of arrow a in FIG. 2, and a stand-by position at which the inner pressing roll 106 is apart from the inner circumferential face of the screen 102 with the roll support member 107 being rotated in the direction of arrow b in FIG. 2. The inner pressing roll 106 is located at the pressing position at the time of printing and is also located at the stand-by position in cases other than the printing process.
Moreover, a doctor roll 109 and a driving rod 110 are respectively installed on the roll support member 107. The doctor roll 109 has a column shape, and is secured to the roll support member 107 in the vicinity of the inner pressing roll 106. The driving rod 110 is supported on the roll support member 107 so as to freely rotate thereon and is placed in an upper space that is formed by the outer circumferential faces of the inner pressing roll 106 and the doctor roll 109 on the respective sides located close to each other. Ink 103 is supplied to this upper space from an ink supplying section, not shown.
Next, an explanation will be given of the outline of the printing operation in succession. A stencil sheet 104 is subjected to a stencil making process by forming perforations in predetermined positions thereof, and the leading edge of the stencil sheet 104 thus subjected to the stencil making process is clamped by a stencil sheet clamp section 100a of the printing drum 100, and attached to the outer circumferential face of the screen 102 forming the circumferential wall of the printing drum 100. Next, the printing drum 100 and the pressure drum 101 are rotated in a direction indicated by an arrow in FIG. 1 in synchronism with each other. Moreover, at the time of printing, the inner pressing roll 106 is allowed to press the screen 102, and in this pressing state, the inner pressing roll 106 is rotated following the printing drum 100. Ink 103, which has passed through the gap against the doctor roll 109, is allowed to adhere to the outer circumferential face of the inner pressing roll 106, and the adhering ink 103 is successively transferred onto the inner circumferential face of the screen 102 by the rotation of the inner pressing roll 106. Moreover, the screen 102 is expanded to the outer circumferential side by the pressing force of the inner pressing roll 106 so that the screen 102 is made in contact with the pressure drum 101.
In this state, as shown in FIG. 1, a sheet of printing paper 111 is transported to the gap between the printing drum 100 and the pressure drum 101, and the sheet of printing paper 111 is successively transported by the printing drum 100 and the pressure drum 101.
The printing paper 111, transported between the printing drum 100 and the pressure drum 101, is further transported while being pressed between the inner pressing roll 106 and the pressure drum 101 together with the screen 102 and the stencil sheet 104. This pressing force allows the ink 103 on the screen 102 side to be transferred onto the printing paper 111 side through the perforations of the stencil original paper 104 so that a printing process in accordance with an image formed on the stencil sheet 104 is carried out.
In the above-mentioned printing operation, the inner pressing roll 106 supplies ink 103 to the inner circumferential face of the screen 102, and also presses the screen 102 so as to exert a pressing force thereon, while pressing the raised portions 114a of the screen 102 at both of the ends of the inner pressing roll 106 so that side leakage of the ink 103 (ink leakage in the center axis direction of the printing drum) is prevented.
More specifically, as shown in FIG. 5, some of the ink 103 located between the inner pressing roll 106 and the rough mesh screen section 102a is not allowed to escape outwards in the axial direction of the inner pressing roll 106, since the inner pressing roll 106 presses the raised portions 114a so as to form a closely contact state between the inner pressing roll 106 and the raised portions 114a. Some of the ink 103 located between the rough mesh screen section 102a and the dense mesh screen section 102b is blocked in its shift outwards in the axial direction of the inner pressing roll 106 by a bonding section 102c so that it is not allowed to escape in the axial direction of the inner pressing roll 106. Moreover, some of the ink 103 located between the dense mesh screen section 102b and the stencil sheet 104 is not allowed to escape outwards in the axial direction of the inner pressing roll 106, since the inner pressing roll 106 presses the raised sections 114a from above so that the pressing force forms a closely contact state between the dense mesh screen section 102b and the stencil sheet 104. With the above-mentioned arrangements, side leakage of the ink 103 on the inner circumferential side as well as on the outer circumferential side of the screen 102 is prevented.
However, as shown in FIG. 6, when the raised portions 114a, etc. are worn out after a long time use, the pressing force of the inner pressing roll 106 applied to the raised portions 114a becomes weaker, and in the worst case, it hardly exists. As a result, since the closely contact state between the dense mesh screen section 102b and the stencil sheet 104 is no longer maintained, the ink 103 located between the dense mesh screen section 102b and the stencil sheet 104 is allowed to escape outwards in the axial direction of the inner pressing roll 106. Then, the amount and spread of the ink 103 escaping in the axial direction of the inner pressing roll 106 increase in proportion to the number of the printing operations, with the result that when the ink 103 has reached the right and left ends of the stencil sheet 104, the ink 103 stains the pressure drum 101, etc. Therefore, because of the ink leakage from the sides of the screen 102, a limitation has to be imposed on the number of prints in endurance operations (the number of prints with guaranteed quality for one sheet of stencil sheet 104).
Moreover, as the wear of the raised portions 114a develops, the ink 103 located between the inner pressing roll 106 and the rough mesh screen section 102a escapes outwards in the axial direction of the inner pressing roll 106 over the raised portions 114a, and since the escaped ink 103 gradually accumulates on the inner circumferential face of the screen 102, resulting in problems such as stains inside the machine due to leakage ink.
Furthermore, in the conventional screen 102, it is necessary to provide the raised portions 114a in addition to the structures such as the mesh screen section 102a and 102b; therefore, the manufacturing process becomes complex with high costs.